The present invention relates to a process for the preparation of nanocrystalline zeolite beta More particularly, this invention relates to the preparation of nanocrystalline zeolite beta by a modified aerogel protocol comprising four steps, namely, hydrolysis, nucleation, crystallization and supercritcal drying. This improved process gives excellent yields of nanocrystalline zeolite beta with a crystallite size in the range of 10 to 80 nanometer and a broad range of silica to alumina ratio 15 to 200 which shows enhanced activity for nitration of o-xylene to produce 4-nitro o-xylene with higher selectivity.
Research has been focused recently on the development of new methods for preparation of zeolites to obtain nanometer size of zeolite crystals. This may be achieved by decreasing the nucleation temperatures, lowering the crystallization times, optimized pH conditions and also in absence of alkali metal cations during the synthesis of zeolites. Zeolite beta, having a three-dimensional large-pore system of a 12-membered ring opening 0.76 nm wide, first described in 1967 in an U.S. Patent, draws much attention because of its unique characteristics, in particular its acidity and potential for acid catalysis. The nanocrystalline zeolite beta offers several advantages over microcrystalline zeolite beta in terms of activity and selectivity due to increased active acidic sites and three dimensional interface with the support and reactant.
Reference is made to U.S. Pat. No. 3,308,069, wherein zeolite beta was described for the first time with a silica-to-alumina ratio from 10 to 150 with crystal size ranging from 0.01 to 0.05 microns in presence of alkali metal cations. The drawbacks are longer crystallization times and also the presence of alkali metal cations makes the zeolite beta inactive acidic catalyst. Reference is also made to Joaquin Perez-Pariente et al, Applied Catalysis, 31,1987,35-64 wherein zeolite beta was synthesized from tetraethylorthosilicate, sodium aluminate, tetraethylammonium hydroxide, sodium and potassium hydroxide. They studied the influence of alkali metal cations on the crystallization mechanism. The drawbacks are the presence of alkali metal cations in the synthetic mixture needs longer post-calcination treatment and zeolite prepared is not an acidic catalyst. Larger crystallites form due to longer crystallization times and separation of zeolite crystals require higher centrifugal forces.
Reference is made to Camblor et al, Zeolites, 1991, 202 and 792, wherein zeolite beta was synthesized in 30 hours at 135xc2x0 C. using amorphous silica, a 40% aqueous solution of tetraethylammonium hydroxide, sodium aluminate, aluminum, sodium hydroxide, potassium hydroxide and suggested that the presence of alkali metal cations is essential for the formation of the zeolite. The disadvantages are the presence of alkali metal cations in the synthetic mixture, formation of larger crystallites and further separation of zeolite crystals requires higher centrifugal forces. Reference is also made to U.S. Pat. No. 5,427,765 wherein zeolite beta is synthesized from a mixture of tetraethylammonium hydroxide, an alkali metal silicate and an aqueous solution containing aluminum. The disadvantages is the presence of alkali metal cations in the synthesis mixture and longer crystallization times required even to form larger crystallites.
Reference is made to U.S. Pat. No. 4,923,690 wherein synthesis of highly silicious zeolite beta was described with silica-to-alumina ratio within the range of 20-1000. The drawbacks are to achieve the high silica to alumina ratio, the zeolite has to be partially crystallized. As the zeolite becomes more crystalline, the silica-to-alumina ratio decreases. In this procedure highly silicious zeolite beta was obtained with only 30 to 50% crystallinity.
Reference is made to U.S. Pat. No. 5,989,518 wherein a continuous process was developed to synthesize various molecular sieves, which control both the particle size and particle size distribution. This process involves continuously adding reactive sources of the desired components along with a structure-directing agent into a continuous crystallization reactor. Either interstage backmixing is introduced or the number of stages is adjusted in order to control particle size The disadvantages are the presence of alkali metal cations and the crystallites obtained are in the range of 3 to 20 microns. Reference is also made to U.S. Pat. No. 5,683,673 wherein zeolite beta is synthesized in presence of ethanol. Ethene is evolved during crystallization period for which the pressure developed autogeneously to 50 bar at the end of crystallization. The drawbacks are the longer crystallization times 11 days at 140xc2x0 C., and separation of zeolite crystals from mother liquor requires higher centrifugation forces of up to 13,000 rpm. Reference is made to M. A. Camblor et al, Studies in Surface Science and Catalysis, Volume 105, 341,1997, wherein nanocrystalline zeolite beta was synthesized with crystallite size 10 to 100 nm in the absence of alkali metal cations by using colloidal silica and aluminium metal powder. The disadvantages are this method requires longer crystallization times and the separation of zeolite crystals from the mother liquor require higher centrifugation forces of up to 16,000 rpm. Reference is made to P. R. Hari Prasada Rao et al, Chemical Communications 1441, 1996 wherein zeolite beta was synthesized by dry gel conversion technique. The drawbacks are this process involves the presence of alkali metal cations in the synthetic mixture, and zeolite prepared will not be an acidic catalyst, longer crystallization times 3-6 days.
The main object of the present invention is to provide an improved process for the preparation of nanocrystalline zeolite beta by a modified aerogel protocol.
It is another object of the invention to provide a process for the preparation of nanocrystalline zeolite beta with a particle size of in the range of 10 to 80 nanometers with Si:Al molar ratio from 15 to 200 wherein the synthetic mixture is free from alkali metal cations.
It is yet another object of the invention to provide a process for the preparation of nanocrystalline zeolite beta where the crystallization times are low and the nanocrystalline zeolite beta is produced in high yield, is highly crystalline and shows the typical beta zeolite IR absorption bands at 575 and 525 cm.xe2x88x921 and X-ray diffraction spectrum.
The novelty of present invention is the preparation of nanocrystalline zeolite beta with the crystallite size in the range of 10 to 80 nanometers and in a broad range of silica to alumina ratio 15 to 200 by a modified aerogel protocol. Most of the previously proposed methods for the preparation of zeolite beta have employed synthetic mixtures containing alkali metal cations and suggested that the presence of alkali metal cations is essential for the formation of the zeolite. Hence the preparation of zeolite beta in absence of alkali metal cations with small crystallite sizes is an exciting process. Controlled hydrolysis of the synthetic mixture, aging at room temperature, reducing the crystallization time and also subjecting the crystalline gel to supercritical drying conditions are novel ideas followed to get nanocrystalline zeolite beta in high yield. The obtained nanocrystalline zeolite beta offers several advantages over microcrystalline zeolite beta in terms of activity and selectivity due to increased active acidic sites and three dimensional interface with reactants for example 4-nitro o-xylene is obtained with higher selectivity in the range of 65-75% from nitration of o-xylene.
Accordingly, the present invention provides a process for the preparation of nanocrystalline zeolite beta comprising hydrolysing a silica source and an aluminium source in the presence of a templating agent and in absence of alkali metal cations, nucleating the resulting product under stirring at room temperature followed by crystallization at higher temperatures and pressures and finally drying the resulting product at supercritical conditions to obtaine nanocrystalline zeolite beta.
In one embodiment of the invention, the nanocrystalline zeolite beta obtained has a crystallite size in the range of 10 to 80 nanometers and a silica to alumina ratio of 15 to 200.
In another embodiment of the invention, the silica source comprises tetraethylorthosilicate (TEOS).
In a further embodiment of the invention the tetraethylorthosilicate is substantially alkali metal free.
In yet another embodiment of the invention, the aluminium source comprises aluminum nitrate.
In yet another embodiment of the invention, the templating agent used comprises aqueous tetraethyl ammonium hydroxide.
In still another embodiment of invention the oxide molar composition is Al2O3:x SiO2:(0.26x+1) TEA2O :15x H2O where x varies between 400 and 14.
In another embodiment of the invention, controlled hydrolysis is effected by the slow addition of aqueous tetraethylammonium hydroxide at room temperature under stirring for 0.25-1.30 hrs
In another embodiment of the invention, the synthetic reactant mixture is aged at room temperature under stirring for a period of 18-48 hrs
In still another embodiment of invention the synthetic mixture is crystallized after addition of hydrocarbon-alcohol mixture in a range of 5:1 to 1:5 moles per mole of SiO2.
In still another embodiment of invention the hydrocarbons-alcohols used for crystallization are selected from the group consisting of hexane, toluene, xylene, methanol, ethanol, butanol and any mixture thereof.
In still another embodiment of invention the crystallization is carried out at a temperature in the range of 120xc2x0 C. to 290xc2x0 C.
In still another embodiment of invention the crystallization is carried out under total pressure of 10 to 100 bar.
In still another embodiment of invention the crystallization is carried out for a period in the range of 1 hour to 5 days.
In still another embodiment of invention, the solvent mixture is vented out at supercritical conditions to obtain a free flow of nanocrystalline zeolite beta.
In still another embodiment of invention, the obtained zeolite beta is calcined in air at 200xc2x0 C. to 600xc2x0 C. for 1 to 24 hours.
In still another embodiment of invention the nanocrystalline zeolite beta shows enhanced activity for nitration of o-xylene to produce 4-nitro o-xylene with higher selectivity in the range of 65-75%.
The present invention also relates to the use of a nanocrystalline zeolite beta obtained by hydrolysing a silica source and an aluminium source in the presence of a templating agent and in absence of alkali metal cations, nucleating the resulting product under stirring at room temperature followed by crystallization at higher temperatures and pressures and finally drying the resulting product at supercritical conditions to obtaine nanocrystalline zeolite beta for production of 4-nitro o-xylene with high selectivity.